In the northern Colorado Front Range, fire suppression during the 20th century is believed to have created a high hazard of catastrophic fire in ponderosa pine (Pinus ponderosa) forests. Since the early 1990s, resource managers have increased the use of prescribed fires to re-create fire regimes and forest structures similar to those of the pre-Euro-American settlement period in order both to reduce fire hazard and to improve forest health. To improve understanding of historical fire regimes, we conducted a study of fire history along an elevational gradient from ϳ1830 to 2800 m in ponderosa pine forests in the northern Front Range. Fire-scar dates were determined from 525 partial cross sections from living and dead trees at 41 sample sites. Fire frequencies and fire intervals were analyzed in relation to changes in human activities and interannual climatic variability as recorded in instrumental climatic records and tree-ring proxy records.Prior to modern fire suppression, the low elevation, open ponderosa pine forests of the northern Front Range were characterized by frequent surface fires, similar in frequency to many other ponderosa pine ecosystems in the West. In contrast, in higher elevation forests (above ϳ2400 m) where ponderosa pine is mixed with Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), the fire regime was characterized by a much lower fire frequency and included extensive stand-replacing fires as well as surface fires. In the mid-1800s there was a marked increase in fire occurrence that can be related both to Euro-American settlement and increased climatic variability. This episode of increased fire left a legacy of dense, even-aged stands in higher elevation ponderosa pine forests, whereas increased stand densities in low elevation forests are attributed mainly to fire exclusion during the 20th century.Warmer and drier spring-summers, indicated in instrumental climatic records and in tree-ring proxy records of climate , are strongly associated with years of widespread fire. Years of widespread fire also tend to be preceded two to four years by wetter than average springs that increase the production of fine fuels. Alternation of wet and dry periods over time periods of 2-5 years is conducive to fire spread and is strongly linked to El Niñ o-Southern Oscillation (ENSO) events. The warm (El Niñ o) phase of ENSO is associated with greater moisture availability during spring that results in a peak of fire occurrence several years following El Niñ o events. Conversely, dry springs associated with La Niñ a events were followed by more widespread fire during the same year.The 1600-1920 fire-scar record indicates that individual years during which high percentages of the 41 sample sites synchronously recorded fire have occurred at least several times per century. The association of these years of widespread fire with very strong ENSO events demonstrates the importance of ENSO-related climatic variabililty in creating extreme fire hazard at a landscape scale.

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The effects of humans and climatic variation on fire history in northern Patagonia, Argentina, were examined by dating fire scars on 458 trees at 21 sites in rain forests of Fitzroya cupressoides and xeric woodlands of Austrocedrus chilensis from 39° to 43° S latitude. Climatic variation associated with fires was analyzed on the basis of 20th‐century observational records and tree ring proxy records of climatic variation since approximately AD 1500. In the Austrocedrus woodlands, fire frequency increases after about 1850, coincident with greater use of the area by Native American hunters. Increased burning, particularly in the zone of more mesic forests, is also strongly associated with forest clearing by European settlers from about 1880 to the early 1900s. The marked decline in fire frequency during the 20th century coincides with both the demise of Native American hunters in the 1890s and increasingly effective fire exclusion. Strong synchroneity in the years of widespread fire at sample sites dispersed over a north–south distance of ∼400 km indicates a strong climatic influence on fire occurrence at an annual scale. Tree ring reconstructions of regional precipitation and temperature show a steeply declining influence of climatic variability on fire occurrence from annual to multidecadal scales. It is the interannual variability in climate, rather than variations in average climatic conditions over longer periods, that strongly influences fire regimes in northern Patagonia. Although climatic variability overrides human influences on fire regimes at an interannual scale, human activity is an equally important determinant of fire frequency at multidecadal scales. Climatic conditions conducive to widespread fire in both xeric Austrocedrus woodlands and Fitzroya rain forests are typical of the late stages of La Niña (cold phase of the Southern Oscillation) events, as indicated by trends in the Southern Oscillation Index and eastern tropical Pacific sea surface temperatures during the 1–2 years before and after fire event years. Years of extreme fire occurrence are associated both with dry winter–springs of La Niña events and with the warm summers following El Niňo events. Years in which the southeast Pacific subtropical anticyclone is intense and located farther south than normal are years of enhanced drought and fire. Similarly, years of widespread fire in northern Patagonia are associated with variations in mean sea level atmospheric pressure at about 50°–60° S latitude in the South American–Antarctic Peninsula sector of the Southern Ocean, as reconstructed from tree rings for AD 1746–1984. Precipitation and, hence, fire regimes in northern Patagonia are significantly influenced by high‐latitude blocking events, which drive westerly cyclonic storms northward. Variations at decadal to centennial time scales in major circulation features, such as ENSO activity and the meridionality of regional air flow at high latitudes, as well as changes in the degree of coupling of these features, influence climate and fire regi...

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This paper examines how rates and mechanisms of succession vary spatially and temporally in xeric, subalpine forests in Colorado, United States. We reconstructed 300 years of succession from limber pine (Pinus flexilis) to Engelmann spruce (Picea engelmanii) and subalpine fir (Abies lasiocarpa) in two watersheds recovering from the same fire. More than 1850 live and dead trees were cored and mapped in 25 plots systematically spaced along a topographic gradient. We used tree ring analysis to reconstruct dates of tree establishment and death. Relative species abundances and basal areas were charted at 20‐yr intervals, and three measures of ecological similarity were also used to capture different elements of change: simple Euclidean distance, Euclidean distance in ordination space, and Horn’s CH similarity index. Successional rates for mesic lower slopes and north aspects were roughly twice those on south‐facing side slopes. Rates were positively correlated with soil P, N, Fe, organic matter, and Parker’s topographic moisture index, and negatively correlated with solar radiation and surface rock cover. Higher rates on mesic plots were primarily the result of heavy overstory mortality of pioneer limber pines ∼200 yr postfire, coupled with higher recruitment of spruce and fir 200–300 yr postfire. On mesic sites, successional rates peaked during this thinning phase of limber pine, but rates were stable or slowly increasing 300 yr postfire on xeric sites. Previous studies have shown that Clark’s Nutcracker catalyzes early succession by caching limber pine seeds in extensive burns, many of which germinate to form multitrunk pines. Limber pines, in turn, may act as nurse trees for spruce and fir. However, we reconstructed past spatial patterns of trees and found that limber pine mortality in midsuccession was strongly related to its clumped pattern of establishment and tendency to attract spruce and fir. Solitary pines with no spruce and fir neighbors within a 2 m radius had a >80% chance of survival, 1893–1993. In contrast, pines within clumps of six or more trees and surrounded by many spruce and fir trees had only 10–20% chance of survival, 1893–1993. The multitrunk growth form of limber pines may be selectively advantageous on harsh sites, including early postfire environments, but our results suggest that this same growth form may be disadvantageous on mesic sites with heavy spruce and fir competition during mid‐ to late‐successional stages. We confirmed the equilibrium prediction that succession is more rapid on mesic sites, but even in this very simple system, demographic mechanisms of succession were relatively complex.